Ta có : M = $\frac{2}{3}$ . 5 + $\frac{2}{5}$ . 7 + ..... + $\frac{2}{97}$ . 99
= $\frac{1}{3}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{7}$ + ..... $\frac{1}{97}$ - $\frac{1}{99}$
= $\frac{1}{3}$ - $\frac{1}{99}$
= $\frac{32}{99}$
Chúc bạn hok tốt ^^